This invention relates to radio location, as of mobile vehicles such as aircraft equipped with transponders, and more particularly to improvements on the invention disclosed in U.S. Pat. No. 4,115,771.
The system disclosed in U.S. Pat. No. 4,115,771 provides essentially all the information that would be available with an actual Secondary Surveillance Radar (SSR) located at the site of the virtual SSR, described in said patent, without transmitting signals and at a fraction of the cost of an actual SSR. Neither the virtual SSR nor the actual SSR are required to remain at fixed locations; their current positions may be determined continuously by known means such as Loran C, enabling the system to operate from mobile bases such as AWACS aircraft, for example.
Under certain conditions that occur rarely and are usually quite transient, the information regarding range of a target craft from the virtual SSR can become inaccurate or indeterminate. In an extraordinary case in which such conditions prevail for more than a few seconds, the situation could be troublesome, particularly if two or more target aircraft at the same altitude are involved.